1. Field of the Invention
The present invention relates to biological production of lactic acid using a microorganism. More specifically, the present invention relates to a transformant being capable of producing lactic acid with a high optical purity at high yield, and a method for producing lactic acid in a convenient and economically beneficial manner using the same.
2. Description of the Related Art
Polylactic acid (PLA) is an aliphatic polyester produced by polycondensation of lactides which is used as a material for bioplastics. Finding important applications in various fields including electronic appliance packing, water bottles, interior materials of automobiles, office furniture, engineering plastics, and fibers, PLA is known for its use as a future material for biodegradable bioplastics. Together with the Global Environment Movement, which has actively been developed worldwide, an upsurge in concern about global warming and climate change compelled the need of environment-friendly, next-generation fiber materials. In consideration of this situation and trend, PLA biodegradable fibers are considered as having extensive potential because they are naturally recyclable, produce no pollution, and have physicochemical and mechanical properties similar to those of synthetic fibers in addition to being biologically degradable. In spite of the potentials, however, biodegradable fibers are little commercialized because they still require production cost 5-˜10-fold higher than that of general synthetic fibers and their mass production is hampered by many difficulties. Further, their uses according to property still remain undeveloped.
Lactic acid (or lactate) has been widely used as a practical commodity chemical in various areas including medicines, food and food processing, cosmetics, chemical substances, etc. In recent years, it has aroused keen interest for its use as a material of the biodegradable polymer PLA (polylactic acid). An upsurge in the demand of lactic acid has provoked the active development of related techniques.
In animals, L-lactate is constantly produced from pyruvate via lactate dehydrogenase in a process of the oxidation of carbohydrates. Lactic acid is an enantiomer in the form of L-(+)-lactic acid or D-(−)-lactic acid. The production of lactic acid may be achieved by a chemical process, but resorts to a biological conversion method using microorganisms in most practical cases.
Commonly, a fermentative production process of lactic acid comprises (1) fermentation, (2) removal of cell mass and proteins, (3) separation and purification of lactic acid, (4) concentration of lactic acid, and (5) dehydration. Fermentative production processes of lactic acid have focused, for the most part, on production techniques by increasing plant scale and production yield through the development of new production processes and the modification of production processes, and on the cost reduction and yield improvement of the separation and purification process by developing excellent separation and purification techniques such as in economically beneficial evaporators and membranes, as well as in dehydration process.
More recently, research and development has been directed to fermentative microorganisms highly advantageous in temperature, pH and organic product productivity, with the advance of various analysis and metabolism engineering techniques. In order to produce lactic acid with high optical selectivity and purity, fermentative processes require higher production costs than do petroleum processes. In addition, fermentative production is economically limited in producing lactic acid of various qualities which have applications in various fields.
Unlike petrochemical processes, fermentative production processes of lactic acid rely absolutely on lactic acid-producing microorganisms. First of all, thus, it is important to secure microorganisms capable of producing lactic acid of high optical purity in terms of mass production capacity and product cost. The mass production of lactic acid with high optical necessarily needs a multi-stage separation and purification process as well as a large amount of raw materials, which results in a rapid increase in production costs. Together with a mass production technique, a microorganism strain capable of lactic acid with high optical selectivity and purity is therefore a prerequisite. In this context, systemic analysis, and metabolism engineering technologies should be provided for optimizing metabolic flux, in combination with a gene allowing for producing lactic acid with excellent optical selectivity.
On the whole, microorganisms used in the production of lactic acid grow at mid temperatures (30-37° C.) and neutral acidity (e.g., pH 7.0). Particularly, lactic acid-producing microorganisms are very sensitive to pH, so that they survive and actively grow only in a narrow pH range while producing lactic acid. In a lactic acid-producing process, the product lactic acid itself is weakly acidic, thus acidifying the environment of the microorganisms. To keep an optimal pH for the fermentation, e.g., a neutral pH, a base such as NaOH, (NH4)OH, Ca(OH)2 and the like, or a carbonate (e.g., CaCO3) is added incessantly. At a pH of 7.0, lactic acid is completely ionized, and thus exists as a salt (lactate). The separation and purification of the lactic acid salt (lactate) into lactic acid needs protonation which can be accomplished with a strong acid such as sulfuric acid. The addition of one mole of sulfuric acid affords two moles of lactic acid with the concomitant production of one mole of a sulfate precipitant (e.g., CaSO4).
Hence, sulfuric acid is required in the same amount (half the amount) as the lactic acid produced. This means with an increase in the recovery and purification of lactic acid, there is also an increase in the expense of sulfuric acid. Besides, the addition of sulfuric acid evokes the problem of causing an expense of precipitate disposal and producing precipitates as an environment pollutant, and thus is economically disadvantageous. There is therefore a pressing need for both a microorganism capable of producing lactic acid even at high acidity (acidic pH), and a technique by which lactic acid or lactate salts can be recovered and purified without producing precipitate wastes.